1. Field of The Invention
The present invention relates to a method for manufacturing a lead frame or substrate, and in particular, to a method for manufacturing a lead frame or substrate for an electronic device such as a DIP or Flat Pack Package wherein a semiconductor or the like is mounted on the lead frame or substrate or the like in the electronic device and sealed in a molding material.
The present invention also relates to a member such as a lead frame and to an electronic device which comprises a lead frame.
2. Description of the Prior Art
FIG. 1 shows the cross section of one part of a conventional electronic device. In FIG. 1, a lead frame 1 is made of a metal such as a copper, 42 ALLOY or the like, and comprises a die 1a for mounting a semiconductor chip 3, and a lead section, that is an inner lead 1b and an outer lead 1c. The semiconductor chip 3 is die-attached e.g. by printing gold onto the surface of the die 1a, so that a gold-silicon alloy is formed at 300.degree. C. to 400.degree. C., and the semiconductor chip 3 is mounted using the gold-silicon alloy as an adhesive 5. In place of the gold-silicon alloy, it is also possible to use an adhesive such as an organic resin of a polyamide type or of an epoxy type, or a silver paste containing an organic material, or the like. An excess 5a of the adhesive is squeezed out during the mounting operation as shown in the drawing. The semiconductor chip 3 and the inner lead 1b are wire-bonded using a gold wire 9, after which the entire package is sealed in a plastic molding 7 with the exception of the outer lead 1c which is to be exposed.
When an organic adhesive is used as the adhesive 5, the adhesive in the flowable state is hardened by a chemical or thermochemical reaction, so that the semiconductor chip is mounted to the die 1a. This method has the advantages that it is inexpensive and easily adaptable to mass production. However, on the other hand, the adhesive 5 and the excess adhesive 5a are gradually vaporized and moved as indicated by the arrow 5b when heated in the range of 100.degree. to 300.degree. C., and an adhering material 5c is formed on the surface of the semiconductor chip 3 and lead frame 1. In addition, natural oxide and a suboxide 15 are formed on the lead frame 1. The adhering material 5c and the suboxide 15 are extremely easy to strip from the surface of the lead frame and therefore end up damaging the adherence of the subsequently-applied molding and protective film to the lead frame 1.
At the rear surface of the lead frame 1, it is easy for moisture to permeate into the plastic molding 7 from outside and to be accumulated thereon. When the device is usually dipped into molten solder, due to heat distortion or sudden expansion of vaporizing water, stress is produced causing the molding to expand. For this reason, cracks 11 and voids 13 are produced at the boundary between the die 1a and the molding 7 in areas where the adherence is poor.
FIG. 9 shows the cross section of one part of another conventional electronic device. In FIG. 9, a lead frame 101, 105 and 102 (reference numeral 102 designates a die to which an IC chip 100 is attached) is made of a metal such as copper, 42 ALLOY or the like. The surface thereof is printed with a good conductor such as gold. The IC chip is die-attached, e.g., by printing gold onto the surface of the die 102, so that a gold-silicon alloy 104 is formed at 300.degree. C. to 400.degree. C., and the IC chip is mounted using the gold-silicon alloy as an adhesive. In place of the gold-silicon alloy, it is also possible to use an adhesive such as a silver paste, a glass insulating material or an organic substance heated to 100.degree. to 400.degree. C. In case of an organic silver paste, an electronic device such as an IC chip is fixed on the die 102 of a metal lead frame with a bond 104 of a silver paste comprising a polyamide or epoxy organic resin and a silver mixed therein. In this case, the electronic device can be fixed on the lead frame by providing a liquid bond 104 between the electronic device and the lead frame and subsequently solidifying the liquid bond 204 by chemical reaction or thermochemical reaction. However, by making a package 103 made of organic resin after the fixing step to enclose the electronic device and the lead frame, an adhering material 106 which easily peels off and a suboxide 107 are produced between the organic resin of the package and the copper or 42 ALLOY. For this reason, moisture permeates into the plastic molding package 103 through the adhering material and the suboxide and collects at the rear surface of the die 102 or the like under usual conditions as well as under severe conditions such as in water or in a high humid atmosphere. This moisture is rapidly vaporized to produce cracks 108 and 109 and a void 110 by soldering at 260.degree. C. for 3 to 10 seconds, for example.
Although a conventional method of fixing the electronic device with organic bond or organic silver paste bond is very suitable for mass production at a low cost, components vaporized from the organic bond are absorbed on the lead frame or the die in an atmosphere at room temperature to 300.degree. C., resulting in a poor adhesion between the lead frame and a molding resin or a protective film.
Further, so-called lead on chip (LOC) structure as shown in FIG. 9 has the advantage that high integration and large scaling of package chips are possible; however, there is the probability that the lead frame undesirably comes in contact with the surface of the chip since the lead frame 101 is located above the chip, that is, there is the probability that short circuit is formed by the contact. In order to prevent the contact, an insulating tape is usually inserted between the lead frame and the chip in the LOC structure. However, the use of such a tape complicates the production process. Further, there is the probability that the chip may suffer mechanical damage from contact between the chip and the insulating tape.